1. Field
The present application for patent relates generally to multi-carrier wireless communication systems, and more specifically to reverse link open loop power control.
2. Background
Communication systems may use a single carrier frequency or multiple carrier frequencies. In wireless communication systems, a channel consists of a forward link (FL) for transmissions from the access network (AN) 120 to the access terminal (AT) 106 and a reverse link (RL) for transmissions from the AT 106 to the AN 120. (The AT 106 is also known as a remote station, a mobile station or a subscriber station. Also, the access terminal (AT) 106, may be mobile or stationary. Each link may incorporate a different number of carrier frequencies. Furthermore, an access terminal 106 may be any data device that communicates through a wireless channel or through a wired channel, for example using fiber optic or coaxial cables. An access terminal 106 may further be any of a number of types of devices including but not limited to PC card, compact flash, external or internal modem, or wireless or wireline phone). An example of a cellular communication system 100 is shown in FIG. 1A where reference numerals 102A to 102G refer to cells, reference numerals 160A to 160G refer to base stations and reference numerals 106A to 106G refer to access terminals.
It is noted that the data rate control (DRC), data source control (DSC), acknowledge (ACK), reverse rate indicator (RRI), Pilot and Data (or Traffic) channels are channels transmitted on the reverse link. The DRC, DSC, ACK, RRI and Pilot are overhead channels. When there is only one DSC on the reverse link carrier, information is provided to a base station 160 for one forward link carrier, the primary forward link (FL) carrier. On the other hand, there may be a plurality of DRC and ACK channels which provide information to a base station 160 for a primary and secondary FL carriers. Also, there will be one RRI and one Pilot channel on each reverse link carrier which provide information on the AT. It is also noted that the FL carriers carry Traffic (or Data) channels and overhead channels such as the ACK channel, the reverse power channel (RPC) and the reverse activity bit (RAB) channel. These overhead channels provide information to the AT.
The system 100 may be a code division multiple access (CDMA) system having a High Data Rate, HDR, overlay system, such as specified in the HDR standard. In HDR Systems, the HDR base stations 160 may also be described as access points (AP) or modem pool transceivers (MPTs). An HDR subscriber station 106, referred to herein as an Access Terminal (AT) 106 and may communicate with one or more HDR base stations 160, referred to herein as modem pool transceivers (MPTs) 160.
An architecture reference model for a communication system may include an access network (AN) 120 in communication with an AT 106 via an air interface. An access terminal 106 transmits and receives data packets through one or more modem pool transceivers 160 to a HDR base station controller 130, referred to herein as a modem pool controller 130 (MPC) by way of the air interface. The AN 120 communicates with an AT 106, as well as any other ATs 106 within system, by way of the air interface. The communication link through which the access terminal 106 sends signals to the modem pool transceiver 160 is called the reverse link. The communication link through which a modem pool transceiver 160 sends signals to an access terminal 106 is called a forward link. Modem pool transceivers 160 and modem pool controllers 130 are parts of an access network (AN) 120. The AN 120 includes multiple sectors, wherein each sector provides at least one channel. A channel is defined as the set of communication links for transmissions between the AN 120 and the AT's 106 within a given frequency assignment. A channel consists of a forward link for transmissions from the An 120 to the AT 106 and a reverse link for transmissions from the AT 106 to the AN 120. The access network 120 may be further connected to additional networks 104 outside the access network 120, such as a corporate intranet or the Internet, and may transport data packets between each access terminal 106 and such outside networks 104. An access terminal 106 that has established an active traffic channel connection with one or more modem pool transceivers 160 is called an active access terminal 106, and is said to be in a traffic state. An access terminal 106 that is in the process of establishing an active traffic channel connection with one or more modem pool transceivers 130 is said to be in a connection setup state.
FIG. 1B is a simplified functional block diagram of an exemplary CDMA communications system. As stated above, a base station controller 130 can be used to provide an interface between a network 104 and all base stations 160 dispersed throughout a geographic region. For ease of explanation, only one base station 160 is shown. The geographic region is generally subdivided into smaller regions known as cells 102. Each base station 160 is configured to serve all subscriber stations 106 in its respective cell. In some high traffic applications, the cell 102 may be divided into sectors with a base station 160 serving each sector. In the described exemplary embodiment, three subscriber stations 106A-C are shown in communication with the base station 160. Each subscriber station 106A-C may access the network 104, or communicate with other subscriber stations 106, through one or base stations 160 under control of the base station controller 130.
Modern communications systems are designed to allow multiple users to access a common communications medium. Numerous multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), space division multiple-access, polarization division multiple-access, code division multiple-access (CDMA), and other similar multi-access techniques. The multiple-access concept is a channel allocation methodology which allows multiple user access to a common communications link. The channel allocations can take on various forms depending on the specific multi-access technique. By way of example, in FDMA systems, the total frequency spectrum is divided into a number of smaller sub-bands and each user is given its own sub-band to access the communications link. Alternatively, in TDMA systems, each user is given the entire frequency spectrum during periodically recurring time slots. In CDMA systems, each user is given the entire frequency spectrum for all of the time but distinguishes its transmission through the use of a code.
In multi-access communications systems, techniques to reduce mutual interference between multiple users are often utilized to increase user capacity. By way of example, power control techniques can be employed to limit the transmission power of each user to that necessary to achieve a desired quality of service. This approach ensures that each user transmits only the minimum power necessary, but no higher, thereby making the smallest possible contribution to the total noise seen by other users. These power control methods may become more complex in multi-access communications systems supporting users with multiple channel capability. In addition to limiting the transmission power of the user, the allocated power should be balanced between the multiple channels in a way that optimizes performance.
A power control system may be employed to reduce mutual interference between the multiple subscriber stations 106. The power control system may be used to limit the transmission power over both the forward and reverse links to achieve a desired quality of service. The reverse link transmission power is typically controlled with two power control loops, an open and a closed loop. The first power control loop is an open loop control. The open control loop is designed to control the reverse link transmission power as a function of path loss, the effect of base station 160 loading, and environmentally induced phenomena such as fast fading and shadowing.
The second power control loop is a closed loop control. The closed loop control has the function of correcting the open loop estimate to achieve a desired signal-to-noise ratio (SNR) at the base station 160. This can be achieved by measuring the reverse link transmission power at the base station 160 and providing feedback to the subscriber station 106 to adjust the reverse link transmission power. The feedback signal can be in the form of a reverse power control (RPC) command which is generated by comparing the measured reverse link transmission power at the base station 160 with a power control set point. If the measured reverse link transmission power is below the set point, then an RPC up command is provided to the subscriber station 106 to increase the reverse link transmission power. If the measured reverse link transmission power is above the set point, then an RPC down command is provided to the subscriber station 106 to decrease the reverse link transmission power.
The open and closed loop controls may be used to control the transmission power of various reverse link channel structures. By way of example, in some CDMA communications systems, the reverse link waveform includes a traffic channel to carry voice and data services to the base station 160 and a pilot channel used by the base station 160 for coherent demodulation of the voice and data. In these systems, the open and closed loop controls can be used to control the reverse link power of the pilot channel.
Initial mobile transmit power is a power control problem when a mobile 106 first establishes a connection with an access point 160. The base station 160 may not control the mobile 106 before it establishes contact with the base station 160. Thus, what power level should the mobile 106 use to transmit its request when initially attempting to access the base station 160? Under the open loop control for single carrier, reverse links as specified in the IS-95 standard, the mobile 106 transmits a series of access probes on the single reverse link carrier when the mobile 106 first attempts to access the base station 160. Thus, the “primary carrier power” is estimated by an open loop control loop.
In a single carrier system, the AT 106 sends an access probe to the AN 120 to access the network 120. Access probes are a series of transmissions of progressively higher power. The mobile 106 transmits its first access probe at a relatively low power, then it waits for a response back from the base station 160. If the mobile 106 does not receive an acknowledgement from the base station 160 after a random time interval, then the mobile 106 transmits a second access probe at a slightly higher power. The process repeats until the mobile 106 receives an acknowledgement in the form of an Access Channel Acknowledge (ACAck) back from the base station 160. Thus, in response, the AN 120 sends an access channel acknowledge signal ACAck. The acknowledgement, ACAck, is received on the access channel. Thus, the initial transmit power for a reverse link traffic channel is determined by the access channel acknowledge signal, and the power level of the corresponding access probe. The system parameter PWR_STEP is the step size for a single access probe correction.